The invention lies in the telecommunications field. More specifically, the invention relates to a method for the compensation of interference in a signal generated by discrete multitone modulation, whereby the signal thus generated has a multiplicity of defined carrier frequencies, and each carrier frequency has a signal vector. The invention further pertains to a circuit configuration for carrying out the compensation method.
Discrete multitone modulation (DMT)xe2x80x94or multicarrier modulationxe2x80x94is a modulation method which is suitable in particular for the transmission of data via linearly distorting channels. By comparison with so-called single-carrier methods such as amplitude modulation, for example, which has only one carrier frequency, a multiplicity of carrier frequencies are used in discrete multitone modulation. Each individual carrier frequency is modulated in its amplitude and phase according to quadrature amplitude modulation (QAM). A multiplicity of QAM-modulating signals are thus obtained. A specific number of bits can thereby be transmitted per carrier frequency. Discrete multitone modulation is used for example for digital audio broadcasting (DAB) with the designation OFDM (Orthogonal Frequency Division Multiplex) and for the transmission of data via telephone lines with the designation ADSL (Asymmetric Digital Subscriber Line).
In the case of ADSL, data are transmitted from an exchange to a subscriber, connected via an analog connection in the telephone network, with the aid of a DMT-modulated signal. In that context, ETSI and ANSI standards stipulate that each carrier frequency has a bandwidth of approximately 4 kHz and transports at most up to 15 bit/s/Hz. The actual number of bits/s/Hz may be different for each carrier frequency, as a result of which the data rate and the transmission spectrum can be adapted to the transmission channel.
A DMT transmission system has a coder which combines the bits of a serial digital data signal to be transmitted into blocks. A complex number is assigned in each case to a specific number of bits in a block. A complex number represents a carrier frequency fi=i/T where i=1, 2, . . . , N/2 of discrete multitone modulation. All the carrier frequencies fi are distributed equidistantantly. T is the time duration of a block. By means of inverse Fourier transformation, the carrier frequencies represented by signal vectors are transformed into the time mode, where they directly represent N samples of a DMT signal to be transmitted. In order to be able to employ inverse fast Fourier transformation (IFFT), a power of two is chosen for N.
After the inverse fast Fourier transformation, a cyclic prefix is implemented, the last M (M less than N) of the samples once again being attached to the beginning of a block. As a result, a periodic signal is simulated to a receiver when the transient process generated by a transmission channel has settled after M samples in accordance with a time Txc2x7M/N. The outlay on equalization in the receiver can be greatly reduced by the cyclic prefix, since after the demodulation in the receiver, it is only necessary to effect multiplication by the inverse transfer function of the transmission channel in order to eliminate the linear distortions of the transmission channel. This requires one complex or four real multiplications for each carrier frequency.
In the case of ADSL, the transmission channel is a two-wire line (copper pair). The two-wire line requires a long time for the transient process in relation to the length of a block. On the other hand, the additional transmission capacity required by the cyclic prefix should be as small as possible.
Given a block length of N=512, a cyclic prefix of M=32 is stipulated in the case of ADSL. However, the transient process of the two-wire line has still not settled after M=32 values. As a result, interference occurs in the receiver, and cannot be eliminated by a frequency domain equalizer.
Such interference can be reduced in the receiver with the aid of particular signal processing measures.
To that end, a time domain equalizer (TDEQ) is connected upstream of a demodulator. The time domain equalizer is designed as a digital transversal filter having adjustable coefficients. The task of the time domain equalizer is to shorten the transient process of the transmission channel. Accordingly, the number of impulse response values of the digital transversal filter must as far as possible be less than the M samples of the cyclic prefix. The design of such time domain equalizers is described by Al-Dhahir and Cioffi in xe2x80x9cOptimum Finite-Length Equalization for Multicarrier Transceivers,xe2x80x9d IEEE Trans. On Comm., Vol. 44, No. 1, January 1996. What is disadvantageous, however, is the high additional outlay on circuitry for the time domain equalizer, caused by the high number of coefficients (between 20 and 40 coefficients), exhibited by the digital transversal filter used as time domain equalizer. A further disadvantage of such time domain equalizers is the high computation complexity, which amounts to approximately 50 to 100 million multiplications per second given a filter length of 20 to 40 coefficients and necessitates a correspondingly high outlay on circuitry. In addition, each coefficient has to be adjusted for adaptation of the digital transversal filter.
It is accordingly an object of the invention to provide a method for the compensation of interference in a signal generated by discrete multitone modulation and a circuit configuration for carrying out the method, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which require a smaller outlay on circuitry than time domain equalizers and can be implemented as a simple and fast algorithm and as a simple circuit, respectively.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of compensating for interference in a signal generated by discrete multitone modulation, wherein the signal generated by discrete multitone modulation has a multiplicity of carrier frequencies each having a signal vector. The method comprises the following method steps:
generating an error signal vector from a reference signal vector, which is a signal vector of the multiplicity of signal vectors in the signal generated by discrete multitone modulation;
assigning to each of the signal vectors of the multiplicity of signal vectors, except for the reference signal vector, a set of adjustable coefficients and multiplying the error signal vector with the adjustable coefficients; and
subsequently adding the error signal vector to each remaining signal vector of the multiplicity of signal vectors for the purpose of compensating for interference.
In other words, an error signal vector is generated from a reference signal vector, which is a signal vector from the multiplicity of signal vectors. The error signal vector is added to each of the remaining signal vectors of the multiplicity of signal vectors for the purpose of compensating for interference. Each of the signal vectors of the multiplicity of signal vectors, except for the reference signal vector, is assigned a set of adjustable coefficients by which the error signal vector is multiplied prior to the addition. In an advantageous manner, the error signal is calculated in one simple step of the method and added to the remaining carrier frequencies in a further simple step. On account of the interdependence of interference of each individual carrier frequency, it suffices to calculate the error signal from a carrier frequency. In contrast to time domain equalization, the method can be designed as an algorithm in a very simple manner.
In accordance with an added feature of the invention, the adjustable coefficients are adapted in accordance with transmission conditions of the carrier frequency having the signal vector assigned to the adjustable coefficients. In an advantageous manner, this adaptation of the coefficients results in better suppression of interference as may be contained in the signal vector.
In accordance with an additional feature of the invention, the adjustable coefficients are adjusted using an iterative algorithm for the purpose of minimizing errors.
In accordance with another feature of the invention, the adjustable coefficients are adjusted with a mean square error algorithm.
In accordance with a further feature of the invention, the reference signal vector is mapped in a value-discrete reference signal vector and the value-discrete reference signal vector is subtracted from the reference signal vector for generating the error signal vector.
With the above and other objects in view there is provided, in accordance with the invention, a circuit configuration for compensating for interference in a signal generated by discrete multitone modulation, the signal generated by discrete multitone modulation having a multiplicity of carrier frequencies in the frequency domain, and each carrier frequency having a signal vector, comprising:
a first decision circuit receiving a reference signal vector, the first decision circuit mapping the reference signal vector into a value-discrete reference signal vector;
a subtractor circuit connected to the first decision circuit for forming an error signal vector by subtracting the reference signal vector and the value-discrete reference signal vector from one another;
a multiplicity of adders receiving the error signal vector and adding the error signal vector to each of the remaining signal vectors, except for the reference signal vector; and
multiplier circuits connected upstream of each of the adders in a signal flow direction, the multiplier circuits multiplying the error signal vector by adjustable coefficients.
In other words, a reference signal vector is fed to a first decision circuit, which maps the reference signal vector into a value-discrete reference signal vector. A subtractor circuit for forming an error signal vector subtracts the reference signal vector and the value-discrete reference signal vector from one another. The error signal vector is fed to a multiplicity of adders, which add the error signal vector to each of the remaining signal vectors, except for the reference signal vector. Multiplier circuits which multiply the first error signal vector by adjustable coefficients are connected upstream of each of the multiplicity of adders.
In accordance with again an added feature of the invention, the adjustable coefficients are adjustable by a manipulated variable. In a preferred embodiment, the manipulated variable is a power of two. Choosing a power of two for the manipulated variable allows the adjustment of the adjustable coefficients to be carried out by means of a simple shift register.
With the above and other objects in view there is also provided, in accordance with an alternative embodiment of the invention, a method of compensating for interference in a signal generated by discrete multitone modulation, wherein the signal generated by discrete multitone modulation has a multiplicity of carrier frequencies each having a signal vector, which comprises the following method steps:
defining a signal vector of a multiplicity of signal vectors in a signal generated by discrete multitone modulation as a reference signal vector, and generating an error signal vector from the reference signal vector;
calculating interference with the remaining signal vectors of the multiplicity of signal vectors in approximation from the error signal vector; and
subtracting the calculated interference from the respective signal vector of the multiplicity of signal vectors for interference compensation.
In this alternative embodiment, the interference in the remaining signal vectors of the multiplicity of signal vectors is calculated as an approximation from the error signal vector, and the calculated interference is subtracted from the respective signal vector of the multiplicity of signal vectors for the purpose of compensating for interference. It is advantageous that adaptive adjustment of coefficients is not necessary in this case. As a result, convergence problems during the adaptation cannot occur either.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for the compensation of interference in a signal generated by discrete multitone modulation, and circuit configuration for carrying out the method, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.